Digital average input current control in power converter
A digital average-input current-mode control loop for a DC/DC power converter. The power converter may be, for example, a buck converter, boost converter, or cascaded buck-boost converter. The purpose of the proposed control loop is to set the average converter input current to the requested current. Controlling the average input current can be relevant for various applications such as power factor correction (PFC), photovoltaic converters, and more. The method is based on predicting the inductor current based on measuring the input voltage, the output voltage, and the inductor current. A fast cycle-by-cycle control loop may be implemented. The conversion method is described for three different modes. For each mode a different control loop is used to control the average input current, and the control loop for each of the different modes is described. Finally, the algorithm for switching between the modes is disclosed.
1. A method for switching between buck, boost, and alternating buck-boost modes of operation of a cascaded buck-boost converter, the converter comprising an inductor, the method comprising:
if the converter is in boost mode and the duty cycle drops below a predetermined value for at least a first predetermined number of cycles, transferring the converter to alternating buck-boost mode;
if the converter is in buck mode and the duty cycle climbs above a second predetermined value for at least a second predetermined number of cycles, transferring the converter to alternating buck-boost mode;
if the converter is in alternating buck-boost mode, and the duty cycle climbs above a third predetermined value for at least a third predetermined number of cycles, transferring the converter to boost mode, while if the duty cycle drops below a fourth predetermined value for at least a fourth predetermined number of cycles, transferring the converter to buck mode;
when the converter operates in a buck mode, controlling the input current according to a pre-programmed buck mode input current control;
when the converter operates in a boost mode, controlling the input current according to a pre-programmed boost mode input current control; and,
when the converter operates in an alternating buck-boost mode, controlling the input current according to a pre-programmed buck-boost mode input current control.
2. A method for switching between buck, boost, and alternating buck-boost modes of operation of a cascaded buck-boost converter, wherein the converter comprises input terminals having input current and input voltage applied thereto and an inductor coupled to the input terminals, the method comprising:
if the converter is in boost mode and the duty cycle drops below a predetermined value for at least a first predetermined number of cycles, transferring the converter to alternating buck-boost mode;
if the converter is in buck mode and the duty cycle climbs above a second predetermined value for at least a second predetermined number of cycles, transferring the converter to alternating buck-boost mode;
if the converter is in alternating buck-boost mode, and the duty cycle climbs above a third predetermined value for at least a third predetermined number of cycles, transferring the converter to boost mode, while if the duty cycle drops below a fourth predetermined value for at least a fourth predetermined number of cycles, transferring the converter to buck mode;
when the converter operates in a buck mode, controlling the input current according to a pre-programmed buck mode input current control;
when the converter operates in a boost mode, controlling the input current according to a pre-programmed boost mode input current control; and,
when the converter operates in an alternating buck-boost mode, controlling the input current according to a pre-programmed buck-boost mode input current control;
sampling current flowing in the inductor;
sampling the input voltage;
sampling output voltage output by the converter;
digitally predicting the input current in a subsequent cycle based on the sampled current in the inductor, the sampled input voltage, and the sampled output voltage according to one of the pre-programmed buck mode input current control, boost mode input current control, or buck-boost mode input current control; and
controlling duty cycle needed for reaching the desired input current at a subsequent cycle.
3. A method for switching between buck, boost, and alternating buck-boost modes of operation of a cascaded buck-boost converter, wherein the converter comprises input terminals having input current and input voltage applied thereto and an inductor coupled to the input terminals, the method further comprising:
if the converter is in boost mode and the duty cycle drops below a predetermined value for at least a first predetermined number of cycles, transferring the converter to alternating buck-boost mode;
if the converter is in buck mode and the duty cycle climbs above a second predetermined value for at least a second predetermined number of cycles, transferring the converter to alternating buck-boost mode;
if the converter is in alternating buck-boost mode, and the duty cycle climbs above a third predetermined value for at least a third predetermined number of cycles, transferring the converter to boost mode, while if the duty cycle drops below a fourth predetermined value for at least a fourth predetermined number of cycles, transferring the converter to buck mode;
sampling current flowing in the inductor;
sampling the input voltage;
sampling output voltage output by the converter;
digitally predicting the input current in a subsequent cycle based on the sampled current in the inductor, the sampled input voltage, and the sampled output voltage; and
controlling duty cycle needed for reaching the desired input current at a subsequent cycle.
4. The method of claim 3 , wherein controlling duty cycle comprises performing a triangle pulse width modulation (PWM).